The motivation for this invention is to determine the magnitude of leaks of natural gas through closed valves in a pipeline. The pressures used in transmission lines for natural gas are sufficiently high that every valve leaks to some extent. The major concern is the magnitude of the leak, since small leaks do not economically warrant repair.
It is not sufficient for the purposes of the industry to know whether or not a valve is leaking. It is known that the valve is leaking. To close down a pipeline for the purpose of repairing a leak in a valve may cost thousands of dollars, and economics may dictate that a repair of the leaking valve is not practical. For example, it may take $10,000 to $25,000 to close down a pipeline and repair a valve. Under circumstances where there is a leak of natural gas through a valve and the gas lost through the leak is worth only $2,000 or $3,000 per year, repair of the pipeline at a cost of $25,000 is not economically practical.
Various inventions have been developed which tell a user whether or not a leak exists in a pipeline. The leak might be in a valve or in the sidewall of the pipe. While this information is necessary, it is not sufficient for deciding whether or not to close down a pipeline to repair a given valve. There is an unmet need in the industry for a means to determine the flow rate of gas through a leaking valve so that informed economic decisions regarding whether or not to repair the valve can be made. With natural gas pipelines a leak through a sidewall must always be repaired immediately for safety reasons. Prior art apparatus which detects and locates leaks in sidewalls is adequate for this purpose.
Gas leaks create sound waves in the gaseous material; more specifically, leaks create sound waves in the gas in a pipeline downstream of the leak. These acoustic pressure waves may or may not be in the audible range, but their magnitude can be measured. In a pipeline, sound waves in the gas excite vibrations in the pipe sidewall, and these also can be measured. This invention measures the magnitude of leaks through a valve by measuring the downstream acoustic pressure waves or pipe wall vibrations. It has been determined that measurement of a particular frequency of waves and summation over a period of time gives a rough measure of the size of the leak. Laboratory experiments with leaks of known size provide a pattern for comparison with acoustic pressure waves measured in the field with a leak of unknown size. The field measurement is compared with laboratory data having known leak rates in similar conditions to approximate the leak rate of the valve in the field.
It will be understood that the invention described herein has a wider scope of use than merely natural gas pipelines and valves therein, but the best mode which will be described herein relates to natural gas. The theory explained below is applicable to both gases and liquids and the term fluids is intended to include both.